Decent, but . . .
If you go on YouTube, there are some videos on the Filter Queen. I, like you, was curious about the Filter Queen after seeing a demonstration and touring the factory in Strongsville, Ohio a few years ago. They are now located in Brooklyn, Ohio.
I decided to add one to my collection. I found one on Ebay for an out-the-door price that had been taken back due to an inability to keep up the payments. It had been cleaned and sanitized and had all the standard attachments. It is the model AT1100 Majestic and is in excellent condition.
It does a decent job in cleaning and has good filtration (per its name).
Here are some weak points about this vacuum. Dumping the dirt out of the basin negates the filtration as dirt flies up. The metal wand does not expand or contract. This is inconvenient when you need a shorter reach, depending out what you are cleaning. You can only use the attachments that come with the machine, as other standard attachments from other vacs cannot be used. There is no height adjustment on the power nozzle. Thus, brushing efficiency can vary depending on type of carpet. The filter cones work, but they are not cheap.
This is an okay vacuum for my collection and how I want to use it. However, I do not believe a brand new Majestic is worth the $3000 price tag they are asking.